Interesting discussion. I cannot make out anything on that gun that's post-55 but pre-metric. Proof in that era required a "tons" marking, which is absent.

I'd really be concerned about that .637 bore. That's a significant overbore for a 20, and I'd be worried about minimum wall thickness.

950 bars is standard modern proof for a non-magnum 20. It'd equate to a service pressure of close to 12,000 psi.